(1) Field of the Invention
The present invention relates to an air bag device arranged at an steering wheel, particularly to an air bag device having a horn switch in a flat planar shape.
(2) Description of Related Art
A conventional or known air bag device having a horn switch with a planar shape (flat plate shape) arranged on a steering wheel is disclosed in Japanese Unexamined Patent Publication No. JP-A-6-305425. This air bag device is arranged at an upper portion of a central boss of a steering wheel and includes an air bag, an inflator, a pad and an air bag holder. The inflator supplies a gas for expansion of a folded air bag, the pad covers the folded air bag, and the bag holder holds the air bag, the inflator and the pad.
The air bag device is arranged with a horn switch having a planar shape with a support plate between the folded air bag and the pad. The support plate supports the horn switch from the lower face side and the support plate is also more rigid than the pad.
The pad is provided with a top wall and a side wall. The side wall is provided with a substantially cylindrical shape extending from the outer periphery of the top wall in the downward direction and is held by the bag holder. Two doors, which open in response to the expanding air bag, are arranged in a portion of the top wall surrounded by the side wall. A thin-walled rupture or tear line having an approximately H-like shape, viewed from above, is arranged about and defines the two doors. These doors are arranged on the front side and the rear side of the steering wheel. The doors cover the folded air bag. Further, the support plate supporting the horn switch is attached to the rear or inside face of the door. The attachment utilizes thermal deformation. Specifically, a plurality of stopper projections extending from the doors are first inserted into predetermined holes of the support plate. Then, the exposed distal ends of the stopper projections are expanded out by heat such that the stopper projections do not come off thereby attaching the support plate to the doors.
According to such an air bag device, during expansion of the air bag, the prearranged rupture portion (or tear line) which surrounds the doors of the pad is initally ruptured by a pushing force exerted by the air bag. As a result, the two doors are opened. Successively, the air bag device projects the air bag from openings of the doors whereby the air bag is expanded significantly.
When an attempt is made to improve the the conventional air bag by incorporating a big horn switch with a planar shape, the following structure results. First, a single door is installed on substantially an entire surface of the top wall of the pad which is surrounded by the side wall. Further, the support plate supporting the planar horn switch is attached to the inside surface of the door by utilizing the above-described thermal deformation technique. However, such a structure suffers from the following disadvantage.
When the door is opened upon expansion of the air bag, a tensile force larger than that in the two doors described in the above publication, is exerted on the door. As a result, the door is elongated considerably. Further, an inertia larger than that in the case of the two doors described in the publication, is imparted on the door. However, the support plate is more rigid and less elongated than the pad. Therefore, the portion of the pad where the support plate is attached (thermally-deformed portion) may be destroyed upon expanding the air bag.
Therefore, the top wall of the pad in the conventional air bag device could not be provided with a large door and arranged with a large planar horn switch at the inside surface of the large door.